The use of foamed thermoplastic materials in the building and furnishings fields involves more and more serious safety problems as far as the flame reaction is concerned.
In particular, the foamed materials of polyolefins having low rigidity (less than approximately 1,400 kg/cm.sup.2), especially ethylene-vinylacetate copolymers obtained by means of an extrusion-foaming process with physical foaming agents, have a very different flame reaction than that of the foamed materials of ethylene homopolymers of low density.
As a matter of fact, in spite of the addition of anti-flame halogenated agents also in synergic combinations with Sb.sub.2 O.sub.3 which provide olefins in general with satisfactory characteristics of self-extinction, it has not yet been possible to achieve self-extinguishing properties in the EVA copolymers used in the production of foamed materials with low density and having the softness required in the building field (sheaths for hot and cold water piping insulation, as well as sheets and slabs for insulation), and also in the furniture industry.
Therefore, the antiflame systems for polyolefins, comprising bromodiphenylethers in combination with antimony oxide, described, for example, in British Patent No. 874,006, are inadequate to impart extinguishing properties to EVA copolymers.
The same applies to the antiflame systems based on halogenated products and organic peroxides, used for styrol resins according to the British Pat. No. 877,864 and U.S. Pat. Nos. 3,058,926 and 3,124,557 as well as the methods described in Belgian Pat. No. 867,873 and in Japanese Application No. 76/134.762, according to which it is possible to obtain foamed polyolefinic products provided with self-extinguishing characteristics by performing reticulation of polymers with peroxides and successively foaming the same with chemical agents.
In the latter cases, it is possible to obtain products of very high density (exceeding 100 kg/m.sup.3), which are non-soft and of higher cost than those prepared according to physical processes of extrusion-foaming.
Furthermore, the decomposition residues of the chemical foaming agents in this case negatively affect the aging resistance of the foamed materials themselves, particularly in contact with metals such as copper, etc.
We have now surprisingly found that it is possible to endow ethylene/vinylacetate copolymers with self-extinguishing properties and at the same time to obtain products suitable for the preparation of soft foamed materials of low density by using an antiflame system comprising one or more bromodiphenylethers containing at least 3 bromine atoms in their molecule, in combination with at least a metal oxide as hereinafter specified, and moreover, at least a compound (activator) having the general formula: ##STR3## wherein: R.sub.1, R.sub.2, R.sub.3, R.sub.4, the same or different, can be hydrogen, aliphatic hydrocarbon radicals with 1 to 6 carbon atoms, chlorine, bromine, phenyl or cyclohexyl radicals, the nitro or the --CN group; R.sub.5, R.sub.6, the same or different, can be hydrogen, chlorine, bromine, aliphatic hydrocarbon radicals containing 1 to 3 carbon atoms, alkoxy radicals containing 1 to 4 carbon atoms, or ##STR4## groups, R.sub.7 being an aliphatic hydrocarbon radical containing 1 to 3 carbon atoms.
U.S. Pat. Nos. 3,441,524 and 3,420,786 have reported the use of diphenyl compounds included in aforesaid formula (I) in combination with bromide organic compounds, to provide the styrene polymers with antiflame characteristics. In this case, the antiflame effect is connected to the property of the diphenyl compounds of modifying the polymer structure at the combustion temperature in a way as to decrease its viscosity. The modified polymer tends to drip at such temperature conditions, with subsequent detachment of the flame from the polymeric material which thus stops burning.
The behavior of said diphenyl compounds in respect to ethylene/vinylacetate copolymers results, on the contrary, to be opposite to the behavior thereof with the styrene polymers in that at the combustion temperature of the copolymer, and generally at a temperature exceeding 200.degree. C., said diphenyl derivatives modify the copolymer structure in a way that causes an increase in the copolymer viscosity and, therefore, at the aforesaid temperature conditions, no dripping of the copolymer occurs.
It is surprising, therefore, that it is possible to obtain antiflame effects in the ethylene/vinylacetate copolymers by the use of the aforementioned diphenyl derivatives.
Thus the object of the present invention concerns the self-extinguishing polymeric compositions comprising:
(a) an ethylene/vinylacetate copolymer having a vinylacetate content of less than 50% by weight; PA1 (b) from 5 to 10 parts by weight, for 100 parts of copolymer, of at least a bromodiphenylether containing at least 3 bromine atoms in its molecule; PA1 (c) from 2 to 4 parts by weight for 100 parts of copolymer of a metal oxide selected from antimony trioxide, bismuth oxide and tungsten oxide, antimony trioxide being presently preferred; PA1 (d) from 0.1 to 1 part for 100 parts of copolymer of at least a compound included in the aforesaid general formula (I).
The ethylene/vinylacetate copolymer preferably contains from 8 to 10% by weight of copolymerized vinylacetate.
Bromodiphenylether is preferably present in amounts ranging between 6 and 8 parts by weight for 100 parts of the copolymer, while the compound of formula (I) is preferably present in amounts ranging between 0.4 to 0.8 parts by weight for 100 parts of the copolymer.
Examples of bromodiphenylethers useful in the practice of this invention are: tribromodiphenylether, tetrabromodiphenyl pentabromodiphenylether, hexabromodiphenylether, triether, bromochlorodiphenylether, tribromodichlorodiphenylether, tetrabromodichlorodiphenylether, octabromodiphenylether, decabromodiphenylether.
Examples of useful compounds (activators) of formula (I) are: 2,3-dimethyl-2,3-diphenylbutane, 2,3-diethyl-2,3-diphenylbutane, 2,3-dimethyl-2,3-di-p-tolybutane, 2,3-dimethyl-2,3-di-p-bromophenylbutane, 2,3-diethyl-2,3-di-p-chlorophenylbutane, 2,3-dimethyl-2,3-di-p-ethoxyphenylbutane, 2,2,3,3-tetraphenylbutane, 1,2-dibromo-1,2-dimethyl-1,2-diphenylethane, 1,2-dibromo-1,2-diphenylethane, 1,2-dinitro-1,2-diphenylethane, 1,2-dinitro-1,2-di-p-tolylethane, 2,3-dicyano-2,3-diphenylbutane, 1,2-dichloro-1,2-diphenylethane and mixtures of said compounds, 2,3-dimethyl-2,3-diphenylbutane being generally presently preferred.
The EVA copolymer utilized preferably has rigidity values ranging between 70 and 700 kg/cm.sup.2, with Melt Flow Index (MFI) values ranging between 0.3 and 4.5 and preferably between 1.3 and 1.8/10 min. and density between 0.92 and 0.95 and preferably 0.928 g/cm.sup.3.
Said EVA copolymers can also be used, for the objects of the present invention, in mixtures with up to 20% by weight, but preferably 5-10% by weight, of polyethylene at low, medium and high density, polypropylene, ethylene-propylene copolymers and ethylene-vinylchloride copolymers.
Anti-oxidizing agents, U.V. and metal stabilizers, lubricants, pigments, nucleants, and inert fillers of the kind conventionally used in such compositions can also be present in such copolymers or mixtures in amounts up to 3% by weight.
The self-extinguishing compositions according to the present invention are preferably granulated before the extrusion-foaming process.
The EVA resin in granulated form is additioned with the antiflame agents and the activators, the synergic agents and all the other additives, such as, for example, the nucleants, lubricants, anti-oxidizing agents, various stabilizers, pigments, antistatic agents, etc., and successively submitted to mixing in a mixer equipped with heating means, e.g., a Banbury mixer.
The homogenized mixture is calendered and the strip of the desired thickness is finally granulated.
In order to obtain good homogenization of the compositions according to the invention, in particular when anti-flame agents such as concentrated "masterbatches" are used, it is preferable to perform cold mixing (for example in a Henschel type mixer) and then to carry out granulation of the mixture by extrusion.
The self-extinguishing compositions thus obtained are particularly suitable for the production of closed-celled, soft, self-extinguishing foamed materials having low density and endowed with good dimensional stability by means of a conventional process of extrusion-foaming with physical foaming agents.
The physical foaming agents appropriate for the extrusion-foaming process are generally constituted by fluorocarbons, in particular dichlorotetrafluoroethane or fluorotrichloromethane, difluoridichloromethane, difluorochloromethane and their mixtures.
The following examples are reported in order to illustrate the invention in more detail and are not intended to be limiting.
For a preliminary evaluation of the flame reaction of the ethylene/vinylacetate (EVA) copolymers compositions with self-extinguishing properties according to the invention and to the examples reported, standard test pieces were prepared, starting from the relevant granulated compositions, by injection molding and were then subjected to the tests reported in the UL 94 specification (Underwriters Laboratories, Inc.).
The test pieces obtained from compositions according to the invention proved to have specified extinction times as provided for the class V O.
The foamed materials obtained by the extrusion-foaming process were, on the contrary, subjected to the flame reaction test provided for in the German specification DIN 4102, Class B.2. The test method under the specification DIN 53438 was used for this evaluation.